As is known in the art, electronic devices (including mobile electronic devices) require highly stable, portable, and energy-efficient reference signal sources which provide a reference signal having a stable output frequency (such reference signal sources are sometimes simply referred to as “reference sources” or “clocks”).
Electronic systems in navigation, telecommunication network synchronization and various sensing (e.g. magnetometry) applications typically utilize high performance clocks to help ensure proper operation. The use of a high performance clock is particularly important in equipment which may operate in environments in which a global positioning system (GPS) signal is not available (e.g. underwater sensors). In portable equipment other clock features such as compact size and high energy efficiency become increasingly more important.
Mechanical oscillators, such as crystal oscillators (e.g. quartz crystal oscillators) and microelectromechanical systems (MEMS) oscillator have been widely adopted to fulfill such demands. Although excellent in short-term stability, such mechanical oscillators suffer from long-term frequency drift due to disturbances from the environment, such as temperature variation and mechanical vibration. This leads to instability well beyond the parts per billion (10−9) level.
To address such problems, atomic clocks have been used. Atomic clocks lock onto the electron transition frequency of atoms. By locking onto the electron transition frequency of atoms, an atomic clock improves the long-term frequency stability. Outstanding long-term stability may be achieved by atomic clocks, such as those using cesium 133Cs, rubidium 87Rb, and hydrogen atoms, which lock the clock signals to electron transition frequencies. Most of these clocks are, however, power hungry and bulky.
Chip-scale atomic clock (CSAC) technology miniaturizes conventional atomic clocks based upon coherent population trapping (CPT). Chip-scale atomic clocks optically probe the hyperfine-state transition of alkali metal atoms and thus can provide excellent long-term stability via a small physical dimension. Using optical coherent population trapping (CPT), chip-scale atomic clocks (CSAC) successfully realized clock miniaturization.
As one example, the SA.45s CSAC achieves a frequency stability (expressed as Allan deviation, (σy) of 3.0×10−10 for a short-term averaging time τ of 1 s and 1.0×10−11 for a long-term τ of 103 s. It consumes a DC power of 120 mW and occupies a volume of 16 cm3.
However, CSACs involve complicated construction of electro-optic components and hence have a high cost, which hinders their wide applications. The hyperfine transitions used in CSACs is also sensitive to external electric/magnetic fields, which therefore necessitates dedicated shielding in the alkali gas cell for long-term stability. Another disadvantage of such CSACs is the long “start-up” time (˜several minutes), which resultant from the alkali-metal evaporation and related thermal stabilization.
Lastly, an electronic servo loop inside a clock typically have a maximum loop bandwidth limited by the absolute linewidth of the spectrum under probing, which is down to kHz-level for CSACs. The resultant low bandwidth (hence slow response time) of the servo loop degrades the capability of correcting fast-changing frequency deviations caused by, for example, clock vibration.
During 1940s to 1970s, the inversion spectrum of ammonia had also been actively exploited as a source of clock references. The inversion of NH3 is essentially vibrational-mode motion, resultant from a Nitrogen atom going through a barrier formed by three Hydrogen atoms under a quantum-mechanical tunneling effect. In 1977, an Allan deviation of 2×10−10 for τ=103 s was reported by using a spectral line (J−K=3−3) of NH3 at 23.8 GHz. Due to the use of a gas cell waveguide with large dimension (half centimeter for single-mode propagation) and length (due to the weak absorption), miniaturization of ammonia clocks is infeasible. Nevertheless, its fully-electronic operation significantly simplifies the clock implementation. And since ammonia molecules remain in gas phase for a wide temperature range, the clock can be turned on from cold instantaneously, which is critical for many real-time applications. Lastly, the higher loop bandwidth due to larger linewidth enhances the stability of ammonia clock under mechanical vibration and shock.